The present invention relates to a process for producing a cordierite-base ceramic honeycomb structure by extruding.
Cordierite-based ceramic honeycomb structures are in use as a carrier for exhaust gas purification catalyst for use in automobiles and various industries, a filter, a heat exchanger, etc.
In recent years, with the progress of technologies, it has been strongly desired to improve the ceramic honeycomb carrier for catalyst used particularly in the exhaust gas system of automobiles. Desired improvements were in (1) volume (smaller volume), so as to obtain better catalytic performance and better light-off performance, in (2) pressure loss (lower pressure loss) for improved fuel consumption and higher engine output, in (3) strength for reduced cost of canning into casing, and in (4) thermal shock resistance and strength for mounting close to engine and resultant higher catalytic activity.
In this connection, investigations have been made to develop a honeycomb structure with thinner ribs and higher cell density, for improved catalytic performance and to produce a honeycomb structure with thinner ribs, having a required cell density, for lower pressure loss. Use of thinner ribs in porous cordierite-based ceramic honeycomb structures produced problems of (a) reduced strength and (b) a substantially increased thermal expansion coefficient caused by the use of die of smaller slit width in extruding and consequent necessity to use finer materials (particularly, magnesia source material).
It was difficult to allow a cordierite-based ceramic honeycomb structure to have a high density.
Particularly, when raw materials were used for cordierite formation having a low thermal expansion coefficient of 2.0.times.10.sup.-6 /.degree.C. or less in a temperature range from room temperature to 800.degree. C. was used, it was necessary to minimize amounts of impurities (which become a flux) such as calcia, alkali or sodium. Consequently, the cordierite-based ceramic honeycomb structure obtained had a very small amount of a vitreous phase and it was porous.
In order to alleviate this problem, attempts were tried using raw materials of low impurity content, for example, severely selected talc, kaolin, alumina, etc. The honeycomb structure obtained, however, had a porosity of as low as 20 to 45%.
It was necessary that a cordierite-based ceramic honeycomb structure, when used as a carrier for an automobile exhaust gas purification catalyst, had a thermal expansion coefficient of 1.5.times.10.sup.-6 /.degree.C. or less.
In the case of a honeycomb structure having a porosity of 30% or less, use of an increased amount of an impurity and fine raw materials were necessary and consequently it was impossible to obtain a honeycomb structure having a thermal expansion coefficient of 1.0.times.10.sup.-6 /.degree.C. or less in a temperature range from room temperature to 800.degree. C.
A cordierite-based ceramic honeycomb structure of relatively low porosity shows a large shrinkage in drying and firing steps and easily generates cracks. Therefore, it was difficult to produce such a honeycomb structure in a high yield and in a large size.
In order to alleviate these problems, JP-B-4-70053 disclosed a method for obtaining a cordierite ceramic having a higher strength by controlling its porosity at 30% or less, i.e. making it more dense.
Thereby, it was possible to prevent, in a honeycomb structure, the reduction in isostatic strength (which is a compressional load applied from the outer wall and circumference of the honeycomb structure), caused by the deformation of honeycomb cells arising during extruding.
In the above method, however, since the cordierite-based ceramic honeycomb structure has a porosity of 30% or less, the honeycomb structure did not have high catalyst coatability; the formability during extruding was not good; and the honeycomb structure had neither sufficiently thin wall thickness nor sufficiently high cell density.